The present application claims priority to Japanese Application No. P11-279790 filed Sep. 30, 1999, which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to a gel electrolyte as a non-aqueous electrolytic solution containing lithium-containing electrolyte salt solved in a non-aqueous solvent which is made into a gel state by a matrix polymer and a gel electrolyte cell using the gel electrolyte.
2. Description of the Prior Arts
Cells are becoming more important industrially as an electric source for portable electronic apparatuses. In order to reduce the size and weight of apparatuses, cells should be light in weight and should not occupy a large space in the apparatuses. For this, the most appropriate cell is a lithium cell having a large energy density and a large output density.
Among the lithium cells, most hopeful are those having a large freedom or flexibility of the shape, or a thin sheet type cell having a large area or a thin card type cell having a small area. However, in the case of a conventional cell using a metal can as a case, it is difficult to produce a thin cell having a large area.
In order to solve this problem, a study has been made on cells using an organic and inorganic solid electrolyte and a gel electrolyte using a polymer gel. In these cells, the electrolyte is fixed, which in turn fixes the thickness of the electrolyte, and there is an adhesive force between the electrode and the electrolyte which can maintain a contact. Therefore, there is no need of using a metal case for containing the electrolyte or applying a pressure to the cell element. Accordingly, it is possible to use a film-shaped case and make the cell thin.
Since the solid electrolyte has an insufficient ion conductivity, it is still difficult to use it in cells. More promising candidate is the gel electrolyte. Its case may be a multi-layered film made from a polymer film and a thin metal film. One of the most hopeful case is a damp-proof multi-layered film made from a heat sealing resin layer and a metal foil layer which can easily be sealed by a hot seal. The multi-layered film itself has an excellent strength and air tightness and is lighter, thinner, and cheaper than a metal case.
However, the non-aqueous solvent in the gel electrolyte cannot constitute a gel electrolyte unless the solvent can be obtain a compatibility with matrix polymer. Moreover, when a film is used as a cell case and the solvent has a low boiling point, if the cell is put under a high temperature, the solvent vapor increase the inner pressure of the cell and may cause swelling. Accordingly, the solvent selection is limited to a small range.
Solvents having a low boiling point used in lithium-ion cells such as dimethyl carbonate, ethylmethyl carbonate, diethyl carbonate, and the like have a high freezing point and a low viscosity, which helps to increase the ion conductivity in the electrolyte at a low temperature. However, because of the solvent selection limit due to the compatibility and boiling point, these cannot be used in a large amount in the gel electrolyte cell using a multi-layered film as a case.
The solvent of the gel electrolyte for cells may be ethylene carbonate, propylene carbonate, and the like as materials having a high boiling point and not causing decomposition which may deteriorate the cell performance. Furthermore, we have developed a matrix polymer having an excellent compatibility with these solvents, excellent chemical stability, gel strength, and excellent liquid-retention characteristic, i.e., a copolymer with polyvinylidene fluoride copolymerized with hexafluoropropylene in a range not greater than 7.5 wt %.
Moreover, when using as a non-aqueous solvent a mixture of ethylene carbonate and propylene carbonate, if the content of propylene carbonate is large, the low-temperature characteristic and the load characteristic are preferable but the initial charge/discharge efficiency is insufficient, which results in a small cell capacity and deteriorates the cycle characteristic. Thus, it has been difficult to produce a gel electrolyte cell which is excellent in all of the cell capacity, cycle characteristic, the load characteristic, and the low-temperature characteristic.
It is therefore an object of the present invention to provide a gel electrolyte excellent in chemical stability, strength, and liquid-retention characteristic and a gel electrolyte cell using the gel electrolyte which can satisfy all of the cell capacity, cycle characteristic, load characteristic, and low-temperature characteristic.
The gel electrolyte according to the present invention includes a non-aqueous electrolytic solution containing lithium-containing electrolyte salt solved in a non-aqueous solvent and made into a gel state by a matrix polymer, wherein the gel electrolyte contains vinylene carbonate or derivative of vinylene carbonate in the amount not less than 0.05 wt % and not greater than 5 wt %.
The aforementioned gel electrolyte according to the present invention contains vinylene carbonate of a derivative of vinylene carbonate in the amount not less than 0.05 wt % and not greater than 5 wt % with respect to the non-aqueous electrolytic solution and accordingly, exhibits an excellent chemical stability of the negative electrode and the gel electrolyte. A gel electrolyte cell using this gel electrolyte exhibits an improved initial charge/discharge efficiency and capacity.
Moreover, the gel electrolyte cell according to the present invention includes: a negative electrode having a lithium metal or lithium alloy or carbon material capable of doping/dedoping lithium; a positive electrode having a composite oxide of lithium and a transition metal and a gel electrolyte arranged between the positive electrode and the negative electrode. The gel electrolyte is a non-aqueous electrolytic solution containing lithium-containing electrolyte salt solved in a non-aqueous solvent and made into a gel state by a matrix polymer, and contains vinylene carbonate or a derivative of vinylene carbonate in the amount not less than 0.05 wt % and not greater than 5 wt % with respect to the non-aqueous electrolytic solution.
In the aforementioned gel electrolyte cell according to the present invention, the gel electrolyte contains vinylene carbonate or a derivative of vinylene carbonate in the amount not less than 0.05 wt % and not greater than 5 wt % with respect to the non-aqueous electrolytic solution and accordingly, the gel electrolyte exhibits an excellent chemical stability with the negative electrode. A gel electrolyte cell using such gel electrolyte exhibits an excellent initial charge/discharge efficiency and satisfies the cell capacity as well as suppresses swelling caused by gas generation and prevent change in cell dimensions and shape.